The present invention relates generally to the field of orthopaedic surgery and has particular applications in the treatment of arthritic joints and to joints replaced with man-made prosthetic joints.
Normal joints in humans and animals are composed of the ends of two bones which are covered by a smooth gliding surface of articular cartilage. The ends of the bones are linked by a fibrous structure called the capsule which helps stabilize the joint and isolates the joint from the surrounding tissues. The inner surface of the capsule is lined with a thin layer of specialized tissue called the synovium, which in response to motion produces a fluid to lubricate the joint. The space occupied by the fluid is referred to as the joint space. Alternating high and low pressures are produced in the articular cartilage and in the joint fluid during normal use, and are important for joint lubrication.
Normal joints allow repetitive smooth gliding motion over many years. Abnormal or excessive wear, however, results in a pathologic condition generically referred to as arthritis (inflammation of a joint). The cardinal signs of inflammation are pain, warmth, redness, and swelling. In general, this pathologic response is associated with a failure of the natural (articular cartilage) bearing surface of the joint, including wear and fragmentation of the cartilage, and the liberation of the wear products into the joint space. These wear products themselves cause inflammation which further contributes to degradation of the cartilage.
Replacement of the diseased and worn joints with man-made (prosthetic) artificial bearing surfaces has revolutionized the treatment of arthritis over the past thirty years. Most commonly, a metal bearing surface is mated to a plastic bearing surface, although other materials and combinations thereof are possible. The specific geometry of the surfaces is dictated by the anatomy and mechanical functions of the particular joint (hip, knee, shoulder, elbow, ankle, etc.). The prosthetic articulation restores a smooth gliding motion to the joint for a number of years. Following the operation to replace an arthritic joint with an artificial joint, tissue reforms around the articulating surfaces. This tissue is referred to as the pseudocapsule, to distinguish it from the natural capsule. Like the normal capsule, the pseudocapsule synovium produces fluid which lubricates the prosthetic articulation.
The coefficient of friction of these artificial joints is about an order of magnitude greater than that of normal articular cartilage against normal articular cartilage. The heretofore known replacement joint materials comprise metals, plastics, ceramics and other materials. These artificial joints wear, releasing particles of the materials from the prosthetic bearing surfaces into the joints. In a mechanical bearing couple, wear of the bearing surfaces is inevitable and occurs to some extent regardless of the type of materials used. The wear particles can interfere with the smooth motion of the joints and further abrade the smooth surfaces of the joint, thus accelerating the wear and destruction of the articulating surfaces.
In addition to these adverse mechanical factors, it has long been recognized that wear particles liberated into the joint space induce a foreign-body inflammatory response which can result in destruction of bone, and may result in loosening of the prosthesis due to deterioration of the contiguous bone anchors [Willert, H.-G. and Semlitch, M., J. Biomed. Mater. Res. 11, 157-164 (1977)].
Because of the relatively large size of the articular surfaces and the increased amount of particulate wear debris that is generated, surface replacements of the hip have demonstrated a relatively high prevalence of periprosthetic bone resorption, loosening and failure as a result of the inflammatory response to particulate wear debris [Bell, R. S. et al., J. Bone & Joint Surgery 67-A, 1165-1175 (1985); Howie, D. W. et al., Clin. Orthopaedics & Related Res. 255, 144-159 (1990); Nasser, S. et al., Clin. Orthopaedics & Related Res. 261, 171-185 (1990)].
The accumulated information indicates that virtually any material in particulate form can incite a foreign-body inflammatory reaction. The degree of the inflammatory reaction may be related to particle size, composition, shape and number. Small particles (generally those that are less than ten microns in size) are ingested by inflammatory cells called macrophages, while the larger particles are ingested by inflammatory cells called giant cells. When a sufficient number of particles have been ingested, specific cellular processes are initiated resulting in an "activated" macrophage [Murray, D. W., and Rushton, N., J. Bone and Joint Surg. 72-B, 988-992, 1990]. Several substances produced by these cells have been demonstrated to be involved in destroying and resorbing the adjacent bone [Goldring, S. R. et al. J. Bone and Joint Surg. 65-A, 575-583, 1983]. A certain amount of particulate debris can be tolerated and not result in this deleterious reaction. However, when the particles are liberated in sufficient numbers and are of a small enough size, it appears that any prosthetic material can cause macrophage activation. Accordingly, it has been observed that artificial joints that are not heavily used (i.e., that have a low wear rate) have low failure rates compared to artificial joints that are extensively used (i.e., that have a high wear rate).
The role that fluid pressure plays in various arthritic conditions and in bone resorption is becoming increasingly clear. It has been found that joint fluid under pressure may erode into bone and result in the subchondral bone cysts seen in osteoarthritis. Furthermore, in normal and prosthetic joints, there is evidence that there are substantial variations in the fluid pressure both in the space containing the articulating surfaces [Hendrix, R. W. et al., Radiology 118, 647-652 (1983)] and in periprosthetic sites at some distance from the articular surfaces [Anthony, P. P. et al. J. Bone and Joint Surg. 72-B, 971-979 (1990)]. This increased fluid pressure can result in fluid and particle transportation over large distances away from the joint, such as into the distal metaphysis of the femur far below a prosthetic hip joint [Pazzaglia, U. and Byers, P. D., J. Bone and Joint Surg. 66-B, 337-339 (1984)]. The particles collect at these sites and incite an inflammatory reaction in the bone. As described above, this inflammation leads to bone resorption (osteolysis) and can necessitate revision surgery.
It is now recognized that in many artificial joints the range of access for joint fluid and particulate wear debris is much greater than the small space surrounding the articular surfaces. This much larger space can include all the bone and soft tissues adjacent to the prosthesis as well as bone and soft tissues at great distances from the articular surfaces. Polyethylene wear debris has even been identified in the iliac and aortic lymph nodes [Charnley, J., Low Friction Arthroplasty of the Hip, New York, Springer-Verlag, pp. 330-331 (1979)]. Thus it is now understood that the joint space can be considered to include all regions, both near and far, that are accessible to joint fluid and particulate wear debris.
While the articular surfaces of a prosthetic joint generally contribute the majority of the particulates, there are other potential sources of particulate debris which include any prosthetic and/or prosthetic-biologic interface where material might be removed by motion and/or corrosion [Svensson, O. et al., J. Bone Joint Surgery 70-A, 1238-1242 (1988); Lombardi, A. V. et al., J. Bone Joint Surgery 71-A, 1337-1342 (1989)]. Because of communication between the space surrounding the articular surfaces and interfacial planes, debris generated in one location may be transported via the fluid around the prosthesis to another location within the joint space.
Hard particles from the prosthetic implant materials and/or bone debris may be transported by fluid through the joint space into the articulation. The wear of the bearing surfaces is accelerated by the presence of particles of a third material interposed between the two articulating surfaces. All bearing surfaces demonstrate substantially poorer performance in the presence of the third material and the wear pattern which results is called three body wear. A hard particle of any size interposed between the bearing surfaces will act like sand in ball-bearings, no matter how well the two surfaces perform under ideal conditions absent the third material. This accelerated wear creates many more wear particles, producing more inflammation and resulting in further bone resorption.
U.S. Pat. No. 4,822,368 to Collier describes the use of a device to contain wear debris around the articular surfaces so that it does not damage the surrounding tissues. It is proposed that the articular surfaces be isolated by a flexible woven polymer enclosure, which (for the hip joint) is attached on one end to the femoral component and on the other end to the acetabular component. Thus, the articular surfaces are isolated from the adjacent tissues.
While this is an attractive concept, it is fraught with practical problems related to the mechanical nature of the enclosure. Even though the enclosure is flexible to allow the motion of the joint, the femoral neck and the acetabular component can impinge on the enclosure at the extremes of motion and the enclosure can be worn and/or torn. In addition, this device does not remove the wear particles from the articular surfaces. Rather, the wear particles already produced will be ground up in the articulating surfaces, giving rise to smaller and more numerous particles which may become small enough to escape through the enclosure. Additionally, the flexible enclosure itself can undergo fretting with repetitive motion (the hip undergoes approximately one million cycles of motion in a year) and thus itself become another source of wear debris. These particles may also act as third bodies, resulting in accelerated wear of the articulating surfaces. Mechanical failure of such an enclosure would result in the release of the contained particulates into the very region which the device was designed to protect. For these reasons, a system which not only collects particulate debris but also effectively removes the particles from the joint space and traps them so that they cannot escape into the tissues is desirable to minimize both the mechanical and biological problems of particulate debris rather than simply containing the wear particles within the articulation by an enclosure member as proposed in U.S. Pat. No. 4,822,368.
In any mechanical bearing system, whether natural or prosthetic, wear is inevitable. New materials and manufacturing processes may result in bearing surfaces with improved wear characteristics. However, no matter what materials are used, wear of a mechanical bearing cannot be eliminated completely. At best it can be kept to a low level.
The present invention provides methods and apparatus generically applicable to any joint, human or animal, natural or prosthetic, which address both the adverse mechanical effects of wear debris and the adverse biological response (generically referred to as inflammation) to particulate debris in pathological states, such as rheumatoid and osteoarthritis, as well as the particulate debris in prosthetic joints. As low levels of wear are tolerated by most joints, it does not appear to be necessary to capture all of the particulates.